1. Field of the Invention
This invention relates to objective lenses of large relative aperture which are capable of high grade imaging performance throughout a focusing range from infinity to extremely short object distances.
2. Description of the Prior Art
The general-purpose photographic objectives are corrected for aberrations so as to provide the best optical performance usually to an infinitely distant object. On the other hand, the macro objectives which aim principally at close up photography has to take account the fact that the aberrations vary with the object distance. In general, the reproduction ratio of 1:10 or thereabout is selected as the starting point of design for which the best optical performance is obtained. In many cases, however, when it comes to large relative aperture objectives, or wide angle objectives, as the image magnification increases, the range of variation of the aberrations is increased. In particular, the spherical aberration and asymmetrical aberrations of extra-axial rays of light are deteriorated extremely badly. This is because the paraxial pencil from a close object tends to diverge more strongly than that from the infinitely distance object so that it is near the stop that the paraxial pencil transits to divergence. Therefore, the spherical aberration of the positive lens group that follows the stop becomes insufficient to compensate for that. Also, since the principal ray of the extra-axial pencil makes a smaller angle with the optical axis, when focusing is effected down to shorter object distances, outward coma is often caused to be produced. Such large variation of the aberrations with the differing reproduction ratios makes it meaningless to optimize the aberration correction at the reproduction ratio of 1/10 or thereabout when the objective is focused on either the infinitely distant object or the extremely close object for which the sufficiently good optical performance is no longer preserved. Particularly in close up photography, it is at a reproduction ratio of about 1/5 that variation of the aberrations begins to increase, causing the imaging performance to be lowered. In order to prevent such loss in the imaging performance due to the change of the reproduction ratio, it has been the common practice in the art to impart a floating movement into part of the optical system. With respect to this type of objective, mention may be made of U.S. Pat. No. 3,884,557, and Japanese Laid Open Pat. No. Sho 50-138823 and No. 53-87728.